#linearaccelerator

This video is about the linear accelerator (or linac as we radiographers tend to call them) that we use to deliver radiotherapy treatment. It is shown delivering a particular technique called VMAT (volumetric arc therapy) but you don’t need to worry too much about the fancy terminology ☺️ I’ve been trying to find a video that shows the linear accelerator that wasn’t too long or goes too in depth - and this is the best I could find I’m afraid! Please excuse the way the video comes across - it’s clearly a marketing clip used by Elekta (manufacturer) but it does show the basics quite well. Elekta is the machine manufacturer that I have trained on and so I feel I can explain it to you pretty well, there is also another major game player in terms of manufacturers - Varian - of which I have heard very good things. I’m so excited about my placement next week at BCCA as they use Varian, which means I’ll get a chance to see both systems, but more on this topic next week 😊 For now I’ll explain some of the terminology used in the video to help you understand how we use the linac. I’ll explain it in the same sequence as the video to hopefully make things a little easier. It might help to pause the video at the beginning once you see the linac so that I can introduce it to you…I hope you’re ready for this - there’s a lot to take it, but it’s worth I promise 😀

Okay, let’s imagine the circle on the wall is a clock face… 12 o'clock you can see the treatment head - this is where the treatment beam of radiation comes from. 3 o'clock - the CT scanner (not exactly what we call it, but this is essentially what it is!). This can take 3d and 4d scans of the patient before (and sometimes after) their treatment. 4d - the fourth dimension being time, and so you can see the tumour move e.g. when the patient breathes - this is mainly used for lung patients in my department. 6 o'clock - is a flat panel that captures 2d images / what many people would call an X-ray. The treatment head works with the opposite panel to obtain this image. 9 o'clock - another flat panel, this one is opposite the CT scanner and captures the 3d and 4d scans. All of these pieces of the linac are on a turning circle called a gantry, so the machine can move 360degrees around the patient (you’ll see this later in the video)

Got it?! 😀 don’t worry if not! It is a complex piece of kit. You can imagine the faces of students on their first day when they walk into a treatment room!

The screens you see either side of the machine display patient information and the exact requirements for their treatment. Then next to the screens are the hand pendants - this is what the radiographer uses to move the linac into the right position.

Right, let’s move on to the rest of video! You can see the table (treatment couch) moves in all directions: closer to and further away from the machine, from left to right and higher and lower. The treatment couch is also on a turning circle so the whole table can spin by 90degrees in the left or right direction. The purpose of the movements in all directions is to enable us to get the patient in the perfect position for treatment.

Next the video mentions mentions gantry speed. This only applies to VMAT (or rapid arc for Varian). It’s basically how fast the machine can move around the patient whilst delivering the patient. This is variable throughout the treatment, so that more dose can be delivered in certain areas and less dose in others. I.e. For some bits of the treatment the gantry will move quickly, for other parts it will slow down. Dose rate is similar to this - changing how much dose the patient receives at given points during the treatment.

Collimators - without going into too much detail..they help to shape the treatment beam to match the tumour/treatment area. Integral dose - not one to worry about too much for basic understanding…it’s the total dose the body receives (not just how much the tumour gets). The radiation beam has to pass through the body to the treatment area in order to reach the tumour and so it’s inevitable that other tissue will her some dose. Radiotherapy works by depositing the highest dose at depth where the tumour is, so the highest dose isn’t at the skin surface like you might think. But I’m not going into that any more, because you know…physics…and I’m not about to bore you with that!

Okay next…here you see the beam coming out of the CT scanner rather than the treatment head. As the video says, it’s low dose to the patient as opposed to the high dose they receive for treatment. This is the same as if you went for a CT scan in the donut machines 😊 The little image on the bottom left shows the CT scan being acquired. We see it on our screens as slices (Google CT image!). This shows the patient in a cross-sectional format so that we can see different levels of internal (and external) organs.

Target - the tumour and any other tissue we need to treat. Organs at risk (OARs) - any important organs close to the target that we want to avoid irradiating to decrease the chance of nasty side effects. Next it mentions VolumeView and table repositioning. This isn’t very clear on the video, but basically what happens is: We compare the original planning CT scan taken before the treatment was planned (called the reference scan) with today’s CT scan from them linear accelerator. These scans are NOT used to see if the tumour is responding to treatment (no1 question we get asked by patients). They are purely used to match up the anatomy between the two images. We want the patient in exactly the same position as they were on the reference scan (as this was used to plan the precise treatment). So, we move today’s image until it’s in the exact same place as the reference image. Any moves are calculated by the computer software and these are converted into treatment couch moves required. So if we moved the image to the left by 0.5cm, then the patient needs to move 0.5cm to the left. This is done automatically from the control room, and so the treatment couch will move in the room before treatment.

Phew! A lot to take in huh?

That’s it for terminology really. The rest of the video clip shows how VMAT is delivered. The smaller screen at bottom left shows how the dose builds up as the machine delivers the treatment. Blue areas are low dose and red areas are high dose (Red in the target volume where we want to be whilst doing our best to avoid those organs at risk - you can see the spinal cord running through the middle is still grey with hardly any dose at all!)

Not all treatments are delivered using VMAT. Some patients will have static beams delivered. So, they may have one from 0degrees, we deliver the beam there and the beam switches off, then we move the treatment head to 90degrees and beam on again. And so on for however many beams are prescribed!

Wow! You made it through to the end!!! Congratulations! 😄

Sorry there’s a LOT of info there, but that’s the best insight into the the technical aspect of my job that I could give you. I wish I had that as a first year student just starting out!!